X-ray tube with magnetic focusing means

ABSTRACT

An X-ray tube having a beryllium window in a metal wall portion of the tube which is integral with the anode, the beam channel extending in the axial direction of the tube at the window being surrounded by a strong magnetic field the lines of force of which extend substantially parallel to the axis of the tube, the region of substantially constant field strength on the side of the tube cross section closed by the window having a maximum width.

I United States Patent 11113,567,983

[72] Inventor Gerhard Heinrich Friedrich de Vries References'CitedEmmasingel, Eindhoven, Netherlands UNITED STATES PATENTS I 1 Appl-733,020 2,692,340 10/1954 Reiniger 250 90 1 Filed May 29, 1968 3,259,7737/1966 Dyke et al 2s0/99x [451 3,374,355 3/1968 Parratt et al. 250/99[73] Assignee U.S. Philips Corporation New York NY. PrimaryExaminer-James W. Lawrence [32] priority June 7 9 7 Assistant ExaminerC.R. Campbell [33] Netherlands Attorney-Frank R. Trifari [3 l 6,708,463

[54] X-RAY TUBE WITH MAGNETIC FOCUSING MEIAS 4D ABSTRACT: An X-ray tubehaving a beryllium window in a 4 C metal wall portion of the tube whichis integral with the anode, [52] U.S. Cl 313/57, the beam channelextending in the axial direction of the tube 313/55, 250/90 at thewindow being surrounded by a strong magnetic field the [51] Int. Cl..H0lj 35/14, lines of force of which extend substantially parallel tothe axis H01 j 35/18 of the tube, the region of substantially constantfield strength [50] Field of Search 313/55, 57, on the side of the tubecross section closed by the window having a maximum width.

PATENTED MAR 2197i X-RAY TUBE WITH MAGNETIC FOCUSING MEANS Thisinvention relates to an X-ray tube provided with an X ray exit windowwhich is made of beryllium and satisfactorily transmits soft X-rayshaving a wave length of not less than A. The window is arranged in themetal wall of the tube. It is known to provide such a tube with means toprevent the window from being struck by electrons. For this purpose, thewindow is screened electrostatically, which is achieved when the windowis at cathode potential or when a gauzelike electrode held at cathodepotential is arranged between the window and the electron target on theanode of the X-ray tube. It is not always possible and with X-ray tubeshaving a metal wall often even undesirable to apply to the window andhence the wall of the tube insulated from the cathode the same potentialas the cathode. Furthermore, it is undesirable to enlarge the distancefrom the electron target on the anode to the window, which is requiredto obtain a sufficient amount of space for accommodating a screeninggauzelike electrode which is at cathode potential.

Electrons striking the anode with a high velocity and then terminatingon the window due to elastic reflection are particularly harmful in anX-ray tube having a window transmitting soft rays, because forwavelengths exceeding 15 A. the thickness of the beryllium window ispreferably not larger than 0.05 mm. so that this window has such a lowheat capacity and such a low thermal conductivity that excessive heatingowing to the energy of the rapid electrons is inevitable, as a result ofwhich such a tube cannot be used for anode voltages exceeding those atwhich X-rays of the said wavelength are produced.

This invention has for its object to avoid this disadvantage. In anX-ray tube according to the invention, the beam channel extending in theaxial direction of the tube is surrounded at the X-ray exit window andon its inner side by a strong magnetic field, the lines of force ofwhich extend substantially parallel to the axis of the tube, while theregion of substantially constant field strength at least on the side ofthe tube cross section closed by the window has a maximum width. In aknown X-ray tube using a magnetic field, this field serves to promotethe concentration of the electrons and the compression of the electronbeams to a minimum cross section. The conventional electrostaticfocusing, which is obtained by suitably shaping the support of thethermionic cathode, is sometimes insufficient and is especiallyunsatisfactory if the electrons are accelerated towards the anode over acomparatively large distance. In such cases, a magnetic focusing fieldis used which preferably has a small focal distance. For this purpose,it is known to provide pole pieces in the immediate proximity of theelectron target in a manner such that a lateral passage for the X-raysis left free between the opposite poles.

Such a lens considerably contributes to the focusing with a maximummagnetic induction along the axis of the lens. The magnetic fieldstrength required is obtained only under the influence of the saturationof the iron of the pole pieces.

Such a magnetic focusing field acts on the elastically reflectedelectrons released in the target on the anode as a stray field becausethe area at which the electrons are produced is located inside theoperative range and in the proximity of the maximum strength of the lensfield. The paths of these electrons, which have a high initial velocity,are not or substantially not influenced by the magnetic field in theproximity of the target. This accounts for the fact that with asufficient strength of the field surrounding the electron paths, aminimum lens effect is advantageous to cause the electrons reflectedfrom the anode to terminate under the influence of the said magneticfield on areas other than the tube window and even to conduct suchelectrons back to the anode.

An X-ray tube according to the invention will be described withreference to the drawing, in which:

FIG. 1 is a sectional view of the X-ray tube according to the invention;and

FIGS. 2, 3 and 4 are a few cross sections of the anode part of the tubeat the level of the window.

A glass sheath 1 encloses a space which is exhausted to a vacuum and inwhich a thermionic cathode 2 is arranged, the support 3 of which isfused to the glass wall. A hollow metal tube 4 projecting beyond a seal5 on the wall 1 terminates opposite the cathode 2. The metal tube 4consists of a ferromagnetic material, for example, iron and supports atits end a copper anode 6 by means of which the tube 4 is closed in avacuum-tight manner. The conventional materials for cooling the anodeare not shown in the drawing. The wall of the metal tube 4 is providedwith an aperture or passage 7 for the X- rays, which aperture is closedby a thin-walled beryllium window 8 which has a thickness of, forexample, about 0.05 mm.

Due to the mechanical and electrical connection between the window 8 andthe anode tube 4, the window is at the same potential as the anode 6.When the cathode 2 and the anode 6 have applied to them the operatingvoltage, electrons elastically reflected from the anode surface 9 willbe deflected towards the window 8 which is a target for such electrons,which are accelerated by the operating voltage applied and the velocityof which is hardly reduced by the deflection, is exposed to strongheating. The electrons are conducted away from the window by a magneticfield which along the beam channel constituted by the inner wall of theanode tube 4 at the level of the window annularly encloses the electronpaths terminating on the anode. Such a strong magnetic field enclosingthe electron paths is capable of varying sufficiently the direction ofthe penetrating electronstravelling towards the window so that suchelectrons pass the window, and strike other areas of the anode tube orreturn to the anode. The desired screening of the window is obtained bya magnetic field of not more than 5,000 Gauss.

Due to the low magnetic induction required to limit the field straying,a thick-walled anode tube 4 must be used which has an annularinterruption at the level of the window 8. When the poles formed due tothe interruption are not excessively energized, the bore of the tube 4is, so to say, magnetically screened so that the field in this space issuppressed. The field is produced by a magnetic yoke 10 which joins theanode tube 4 and is provided with an energizing magnet coil 11.Alternatively, the required field may be produced by a permanent magnet.The interruption 12 provided in the anode tube 4 for forming the magnetpoles is suitably bridged by a nonmagnetic material.

In order to ensure that the field is located as close as possible to thewindow 8, it is efficacious to increase the wall thickness of the anodetube on this side. The cross sections of FIGS. 2 and 3 are shaped sothat, while retaining a sufficiently strong field at the window, theanode tube may have a smaller weight. The shape of the cross section ofFIG. 4 is preferred when two or more windows 8 transmitting Xrays invarious directions are arranged, as is the case, for example, inmultiple X-ray diffraction apparatus.

It has been found that the dimensions of the tube may be substantiallyequal to those of the construction without the window screeningdescribed.

We claim:

1. An X-ray tube comprising an evacuated envelope having a metal wallportion provided with a window therein for the transmission of X-rayshaving a wavelength of not less than 15 A., a cathode and an anodewithin said envelope defining a given axis within said envelope, saidanode being integral with said wall portion having said window, andmeans in proximity to said anode for generating a magnetic field thelines of force of which extend substantially parallel to the axis ofsaid tube, said window being positioned parallel to said axis wherebysaid window, in the absence of the magnetic field, is struck byscattered electrons from said anode, said magnetic field having a regionof substantially constant field strength of maximum width in proximityto said window whereby scattered electrons from said anode are preventedfrom striking said window.

2. An X-ray tube as claimed in claim 1, wherein the inner and the outercross section of the metal wall are both circular and are eccentricallyarranged with respect to each other,

4. An X-ray tube as claimed in claim 1 wherein the wall portion providedwith the window is annular and has a larger thickness.

2. An X-ray tube as claimed in claim 1, wherein the inner and the outercross section of the metal wall are both circular and are eccentricallyarranged with respect to each other, while the X-ray exit window isarranged in the thicker wall portion.
 3. An X-ray tube as claimed inclaim 1, wherein the metal wall has an inner square cross section and anouter circular cross section.
 4. An X-ray tube as claimed in claim 1wherein the wall portion provided with the window is annular and has alarger thickness.